The present invention relates generally to electric motors and their manufacture, and more particularly, to methods for casting rotors for electric induction motors.
One form of electric induction motor involves a rotating armature (rotor) surrounded by a coil-wound stationary field (stator). When electric current is passed through the stator windings, a part of the stator known as the pole (which may be made up of a magnetically permeable material, such as iron) around which the windings are wrapped becomes magnetically energized, which in turn imparts an electromagnetic force to the rotor, causing it to rotate. In motive applications, a shaft attached to the rotor can be used to provide propulsive force to a vehicle through the turning of one or more linked wheels. Such a motor could be especially useful in vehicles that rely either entirely on electric power, or as part of a hybrid system, where the electric motor and an internal combustion engine (such as conventional gasoline or diesel engine) cooperate with one another to produce the motive force.
A “squirrel-cage” rotor is a common example of an electric induction motor, and derives its name from its cage-like cylindrical shape, where numerous metal rotor bars or rods extend longitudinally and are spaced around the cylindrical periphery of a central axis of rotation. The bars are held in a fixed relationship to one another by metal end rings so that adjacent bars and connected end rings form numerous coil-like electrically continuous loops. Due to the proximity of the rotor to the stator, changes in the magnetic field produced in the stator induce current in the highly conductive loops formed by the bars and end rings. This current turns the rotor into an electromagnet that can spin in an attempt to align itself with the magnetic field produced in the stator. To increase the magnetic intensity of the rotor, a series of laminated plates (typically made from a material that has a lower magnetic resistance (i.e., more magnetically permeable) than air, such as iron) are mounted to the shaft or related mandrel such that they occupy the substantial entirety of the space between the shaft and the cage formed by the bars and end rings. Typically, an interlocking stamping process or loose laminations could be used to secure each of the plates together. Moreover, a low electrical conductivity material (for example, a coating) could be used to minimize electrical contact between them. The cooperation of the laminated stack of plates with the current flowing through the loops of the cage help to strengthen the magnetic field generated by the loops of the rotor, and leads to higher levels of torque generated in the attached shaft. To keep the torque generated at a relatively constant level, the bars making up the cage may be skewed to define a slightly helical pattern rather than one that is strictly longitudinal. In one form, the bars and end plates are separately-formed structures that are joined together through well-known techniques. As with the connection between the plates of the laminate stack, a non-conducting adhesive may be used to secure the bars to the slots of the laminated plates.
In another common form, the longitudinal metal bars may be cast directly into the slots once the laminate plate structure has been assembled. Casting of a squirrel-cage rotor is advantageous relative to assembling it from separate parts, as it reduces the cost and manufacturing variances associated with assembled components. As with the formed bars discussed above, it is desirable to make the cast bars from a high electrical conductivity material, such as copper or aluminum. The manufacture of the rotor cage through casting has traditionally been done by high pressure die casting or squeeze casting. With die casting, molten metal is forced under high pressure into reusable die molds that are typically made from a tool-grade steel. This process is well-known, and is relatively inexpensive. With squeeze casting, the molten metal is injected at a lower, less turbulent velocity, with higher pressures, into a die, and is also typically made from a tool-grade steel.
Unfortunately, either of these forms of squirrel-cage rotor casting suffers from drawbacks. In particular, the cross-sectional dimensions of the passages formed in the laminate stack that define the slots for the longitudinal bars are typically very small (for example, on the order of 2 millimeters), thereby requiring rather large pressures (typically between 2000 and 5000 pounds per square inch gauge (PSIG)). Such pressures, while promoting fast (on the order of a tenth of a second) fill time, also result in high molten flow turbulence and related gas entrapment in the relatively long but narrow passages and in the end rings. Of particular concern is the increased porosity of the bars and end rings that make up the rotor's cage, especially at the remote end from the molten metal injection site. Because the performance of squirrel-cage rotors is closely related to the electrical continuity between the bars and the end rings, such porosity is undesirable. Equally problematic is the impact that porosity has on the mechanical properties of the cast rotor, again especially at the end remote from the mold gating and related fluid introduction. With a rotor diameter of up to approximately eight inches spinning at speeds of between 10,000 and 15,000 revolutions per minute (RPM), a porous end ring will be more likely to fail than a fully dense one.
As mentioned above, both die casting and squeeze casting involve the use of steel molds, while substantially pure aluminum, with its combination of high electrical conductivity and low cost relative to copper, is frequently used as the casting material for the squirrel-cage of the rotor. When such aluminum in molten form comes in contact with the tooling-grade steel of a die cast or squeeze tooling die set, it aggressively attacks the iron in the die. As such, the useful life of such die sets (which are expensive to replace) is rather limited.
A more recent alternative has been to use semi-solid metal (SSM) casting. Instead of using liquid metal, the SSM casting process uses metal that is partially solid and partially liquid, where the consistency of the metal allows it to be injected into dies at relatively low pressures. SSM, while reducing the likelihood of porosity in the finished part relative to the methods discussed above, involves complex mixing or shaking to take advantage of the material's thixotropic properties, and is therefore expensive. Furthermore, SSM may be very difficult to control with high purity aluminum, copper or alloys thereof, which often have very limited solidification ranges.
Another problem with conventional casting techniques is that the molten metal in any one of the passages has a tendency to solidify (i.e., freeze) prior to passing through the narrow passageways formed in the laminate stack and into the remote end ring. This tendency is exacerbated when the temperature of the laminate stack is relatively low, as such tends to act as a heat sink for the flowing molten metal.
Accordingly, it would be desirable to provide a casting method for forming squirrel-cage rotors for induction motors that reduces the occurrence of porosity and the related reduction in the mechanical and electrical properties of the rotor. It would also be desirable to keeping the cost of such casting low.